Apparatus for making annularly ribbed plastic pipe and method of making such pipe

ABSTRACT

An apparatus for forming a thermoplastic pipe having a profiled surface includes a nozzle having an extrusion orifice feeding extrudate in a downstream direction to a mold path which extends from the nozzle and along which are provided a first mold block and a second mold block downstream of the first mold block. The mold blocks are closable around the extrudate to form the pipe and are openable to release the pipe. The mold blocks are moved together with one another and with the pipe in the downstream direction when the mold blocks are closed and are opened and sequentially moved in an upstream direction along an axis contiguous with the axis of the mold path while the extrudate continues to move in the downstream direction. The first mold block is moved in the upstream direction before the second mold block such that the mold blocks never pass one another on the mold path. After the mold blocks are moved in the upstream direction, they are then reclosed around the extrudate at a position to maintain consistency of the profile of the pipe.

This invention relates to apparatus and a method for making annularly orhelically ribbed plastic pipe and method of making such pipe.

Semi-rigid thermoplastic, annularly profiled pipe such as corrugated orribbed pipe or substantially smooth walled pipe, has traditionally beenmade in a travelling mold tunnel. Conventional travelling mold tunnelscomprise two endless tracks of hot mold blocks which come together alonga forward run of the tunnel to provide a cylindrical mold travelling inthe direction of extrusion. Such travelling mold tunnels with twoendless tracks of half mold blocks are complex pieces of machineryrequiring precise synchronization of the tracks. Moreover, they requirea very great deal of space to accommodate the return runs of the tracks.This use of space becomes excessive when large diameter pipe is beingmolded. To best utilize the space available some conventional travellingmold tunnels have been made with one run above the other rather than inside by side relationship.

Various attempts have been made to provide travelling mold tunnels ofother types. Thus, for example, Manfred A. A. Lupke in U.S. patentapplication Ser. No. 854,988 directed to hinged mold blocks, provides anapparatus in which mold blocks are hinged together and follow a returnpath in the same plane as the operating run without the need to changethe orientation of the mold blocks during their return.

Another example of an early type of travelling mold tunnel which doesnot involve a return run along a different path is disclosed in CanadianPatent No. 962,412 issued Feb. 11, 1975 to Industriele Orderneming WavinN.V. In that Canadian patent there is disclosed a mold tunnel whichcomprises only two pieces. Each piece is a longitudinal half of thewhole tunnel. In operation the two piece mold is closed and movesforwardly at extrusion speed. At intervals it opens and moves rearwardlyso that they never advance so far forward as to leave the moldingregion. That type of arrangement involved accurate precise movement ofheavy long mold halves. It was necessary that he halves have appreciablelength to support the pipe molded therein during cooling and setting.

A later mold tunnel of a somewhat similar type is disclosed in U.S. Pat.Nos. 4,911,633 and 5,017,321 issued Mar. 27, 1990 and May 21, 1991respectively to Gordon A. Comfort. These patents disclose a method andapparatus for using a small number of mold blocks, e.g. two or threemold blocks in a mold tunnel. Each mold block is divided into twolongitudinal halves. As one mold block, with its halves closed inmolding configuration, advances the other (or others) leapfrog over iton a second parallel path to take up an upstream position. Such anarrangement allows for smaller, higher, more maneuverable mold blocksthan those of Wavin, but it does not provide for return of the blockswithout displacing them from the production track.

The present inventor has addressed the problem of still further spaceconsideration. He has also addressed the problem of opening the moldblocks without the need for hinging in a system where mold block returnis without change of orientation of mold blocks themselves. He has stillfurther addressed the problems that exist in returning the mold blocksalong the same path as that of their advance.

He has, therefore, attempted to provide a system in which the returnpath of the mold blocks is not laterally displaced from the operatingpaths.

According to the invention there is provided apparatus and a method formolding profiled, thermoplastic pipe. The apparatus comprises Atravelling mold tunnel for the molding of pipe therein in an operatingrun, the mold tunnel comprising at last two mold blocks each having atubular mold surface in a closed condition and being openable into anopen condition to release pipe molded therein, in which individual moldblocks of the mold tunnel are returnable in steps at regular intervalsalong a return path whose longitudinal axis is contiguous with thelongitudinal axis of the operating run. The mold blocks thus move in aback and forth linear manner, never passing one another. Thus whilethere is an overall constant movement forward in the extrusiondirection, individual mold blocks pulsatingly backstep to maintain aposition within a specific distance with respect to an extrudatedelivery point into the tunnel. Usually the tubular mold tunnel iscylindrical but other tubular forms are contemplated.

More particularly, the invention comprises apparatus for formingprofiled thermoplastic pipe comprising; at least two mold blocks, eachblock being closeable into a closed condition to form a tubular moldsurface and openable from said closed condition, each block beingmoveable along the track in forward and rearward directions along anaxis of the cylindrical mold surface; an elongate extrusion nozzlehaving an annular extrusion orifice projecting into an upstream, end ofthe travelling mold tunnel along the axis of the cylindrical moldsurface for the extrusion of extrudate into an upstream one of the moldblocks thereinto; means to move the mold blocks together in forwardtravel along the axis of the cylindrical mold surface at a forward speedcommensurate with mold of pipe in the mold tunnel; means to pulseindividual mold blocks in rearward travel against the direction of theforward travel, the mold blocks being in their closed condition duringforward travel and in their open condition during rearward travel; andmeans to initiate said rearward travel sequentially for each mold blocksuch that an upstream mold block starts rearward travel before a nextadjacent downstream mold block.

The upstream mold block (herein called the first mold block), i. themold block which receives hot extrudate from the extrusion nozzle,should have an axial length greater than the distance of return travel.However, the distance of return travel may be quite short.

Thus, the upstream mold block performing the primary molding ofextrudate from the extrusion nozzle may be correspondingly short. Theannular extrusion orifice may be a single annular orifice or maycomprise coaxial orifices for the production of double-walled pipe.

It is believed a potential advantage that the upstream mold block be asshort and light and maneuverable as possible, for example that thebackward pulse of the mold block may only be sufficient to relocate amolding profile e.g. an annular crest, from one corresponding trough ofextrudate into the next adjacent upstream trough. Of course, thebackward step or pulse may jump several profiles. Whether the backstepor pulse covers only one profile or several, it is necessary that thebackstepped mold blocks re-close over formed extrudate at a point wherethe mold profile of the block and that of the formed extrudate match.Thus, it is thought desirable that the regular intervals of thebacksteps should be frequent, and the distance of travel of thebacksteps should be short.

Indeed, it is important that the point at which extrudate issues out ofthe extrusion nozzle into the travelling mold tunnel is always within aclosed part of the tunnel. Thus the length of a mold block must begreater than that of the predetermined distance through which it travelsin a forward and reverse direction. When the mold block into which theextrusion nozzle projects is at its extreme upstream position, i.e. atthe beginning of its forward travel, the orifice of the extrusion nozzlemay be in a downstream region of the mold block. As the mold blocktravels forwardly over the extrusion nozzle orifice, it will move suchthat the extrusion nozzle orifice becomes located in an upstream regionof the mold block. Before the mold block travels so far forwardly thatthe extrusion nozzle is exited from the mold block, the reverse movementmust take place.

The next downstream mold block (herein called the second mold block)from the upstream mold block may be of the same length as the upstreammold block or different. While all mold blocks may carry services, suchas cooling means for the pipe, this next downstream mold block may belonger than the upstream mold block and may be especially useful forcarrying means for cooling the extrudate. Cooling in any or all the moldblocks may be by water cooling or, in some cases, air cooling.

In another mold of operation two shorter mold blocks may togethercomprise a composite mold block and move together.

If more than two mold blocks are used, e.g. three mold blocks, then thefurther downstream mold blocks may again be shorter than the second moldblock. It is an advantage to provide a mandrel support within the pipebeing formed. When the first mold block opens to release the soft formedextrudate, it will move backwards only a short distance and may closeagain before the extrudate has had an opportunity to deform.Nevertheless, support within the formed extrudate at this point may beof appreciable advantage. The second and further downstream mold blocksmay have the effect of finishing or refinishing any anomalies in thepipe due to momentary release of the mold. Such anomalies may, in anycase, be no worse than marks on the pipe produced between adjacent moldblocks of a conventional travelling mold tunnel.

It will, of course, be appreciated that the back and forth pulsationsufficient to allow for continuous forward production speed of theextrudate without allowing it to sag out of shape. Normal extrusionspeeds (and thus the speed of the mold tunnel) may be in the range of 1to 2 meters per minute for large diameter pipe.

Any number of mold blocks may be used in the mold block train. As,in anytravelling mold tunnel, the upstream mold block in the operating run aremost active in the preliminary molding process and the furthestdownstream of the mold blocks playing a more important part insupporting molded pipe while it cools. It may be that two mold blocksare sufficient in the present apparatus, the first, upstream mold blocksurrounding the orifice of the extrusion nozzle and reciprocating with aslow forward stroke and a fast return stroke to play the most activemolding part. The second mold block of this pair will act to supportpipe especially during the return stroke of the first mold block.further downstream mold blocks will provide further backup.

It is probable, however that a preferred number of mold blocks may bethree.

Of course, a number of blocks greater than three may be used but it isthought that additional apparatus parts and equipment may not providemuch significant advantage over the use of three blocks.

Conveniently each mold block is carried on carrier blocks. The carriermold blocks may be moveable forwardly and rearwardly on a longitudinaltrack and each mold block may comprise a pair of diametric mold blockhalves reciprocally slidable on the respective carrier block transverseto the direction of movement of the carrier block to open or close themold block. Each carrier block may have a groove and tongue connectionwith the base of the mold block which it carries, i.e. the bases of bothmold block halves making up said mold block. Each carrier block may bewider than its associated mold block so that the mold block halves mayslide on their groove and tongue connections with the carrier blocks toclose to form the closed tubular mold block and to open when they slideapart.

Although the invention is generally described with reference toapparatus including carrier blocks, it will be appreciated that othermeans for carrying and aligning the mold blocks are possible, forexample guide carrier rods may be used in place of the carrier blocksand any track therefor.

Opening and closing of the mold blocks may be mechanically achievedthrough hydraulic mechanism initiated through a sensor switch when themold block reaches either a predetermined position for opening orclosing respectively. Alternatively, opening or close may be through acam mechanism or any other mechanism within the skill of a man skilledin the art. Return mechanism for moving each mold block on its returnstroke may be any convenient mechanism such as a cam mechanism orhydraulic mechanism dependent on the means chosen to drive the moldblocks. In effect the mold blocks are disengaged from a continuousforward drive and engaged with drive for the back step. The mold blockhalves may slide fully apart or may open in a hinged manner or in anyother convenient manner.

The positioning of the mold block at the beginning of the forward strokeis of importance. If ribbed or corrugated pipe is being formed it isimportant that the mold block is positioned exactly so as to providecontinuity in the pipe. Thus, when positioning the mold block at thebeginning of the forward stroke, at last the most recently or upstreamcorrugation of already molded pipe should mate with the appropriatedownstream molding surface of the mold block. Such precise positioningof the mold blocks in relation to the pipe may be carried out by anyconvenient means.

BRIEF DESCRIPTION OF THE DRAWINGS

An embodiment of the invention will now be described by way of examplewith reference to the drawings in which;

FIG. 1 is a perspective view of one mold tunnel according to the presentinvention;

FIG. 2 is a view of one closed mold block of the mold tunnel of FIG. 2;

FIG. 3 is a view of the mold block of FIG. 3 open;

FIGS. 4A to 4G indicate positions of the mold blocks of a mold tunnelsuch as that of FIG. 1 during different parts of the cycle in moldingpipe; and

FIG. 5 shows another sketch of the mold tunnel with mold blocks ofdifferent length.

DETAILED DESCRIPTION ACCORDING TO THE PREFERRED EMBODIMENTS OF THEPRESENT INVENTION

FIG. 1 illustrates mold tunnel 10 moving in a track 12 in the directionof arrow A. An extrusion nozzle 14 extends into the mold blocks 16A, 16Band 16C of the extrusion tunnel 10 from extruder 18 which is shown onlyroughly. The extrusion nozzle 14 has a delivery end 20 for extrudatelocated within the mold tunnel 10 to mold pipe 26 therein.

Each of the mold blocks 16A, 16B and 16C is made up of two mold blockhalves which are slidable towards each other and away from each other oncarrier blocks 22 which are movable in either direction on guide track12. Each carrier block has a ridge or tongue 24 on its upper surfaceextending laterally at 90° to the axis of the track 12. This tongue 24co-operates slideably with a groove in the bottom of each of the moldblock halves of mold blocks 16A, 16B and 16C so that the mold blockhalves may slide together and apart.

When the mold block halves slide apart they disengage from pipe 26. Whenthe mold block halves are slid together they mate one with the other toform a tubular mold block at a mating diametric plane 27. Suction may beapplied, if desired, at the mold surface of each mold block 16A, 16B,16C to enhance molding. Such suction may be applied by conventionaltechniques which will not be described here.

Any convenient mechanism may be provided for causing the carrier blocks22 and their associated mold blocks 16A, 16B and 16C to move in thedirection of arrow A (See FIG. 1 and FIGS. 4A-4G). For example, anendless chain, the path 28 of which is indicated in FIG. 1 in brokenlines, engaging drive lugs 30 of the carrier blocks may be provided or atoothed ratchet mechanism may be provided. This is largely a matter ofchoice and no especially preferred means has been illustrated. A one wayratchet mechanism may, however be desirable so as to provide for easyreturn of the carrier blocks on the return stroke. Whatever the drivemeans chosen, it should be possible to transport the carrier blocks 22and their associated mold blocks 16A, 16B and 16C in the direction ofarrow a at a speed conforming with the speed of extrusion, i.e. theproduction speed of extrudate from the extrusion orifice 20 of theextrusion nozzle 14.

In the embodiment shown three mold blocks 16A, 16B and 16C are used.This is not intended to be limited and the use of more mold blocks ispossible and the use of only two mold blocks is also possible. Furtherfeatures of the apparatus will become apparent from the followingdescription of the operating cycle in forming annularly profiledthermoplastic pipe, for example annularly ribbed or corrugated plasticpipe. The production of helically profiled pipe is also possible and, inthis specification and claims, the term "annularly" is intended toinclude helical.

In operation, the carrier blocks are moved forward in the direction ofarrow A at a constant speed dictated by the speed of extrusion throughextrusion nozzle 14. At the beginning of the forward movement thedelivery orifice of extrusion nozzle 14 is located as shown in FIG. 4Aand the mold blocks 16A, 16B and 16C are generally in the positionsshown in FIG. 4A. In all of FIGS. 4A to 4G the mold blocks 16 arevariously labelled "C" or "O". A "C" designation indicates that the moldblocks are closed for molding tube and an "O" designation indicates thatthe mold block is open for its return stroke.

As the mold blocks 16A, 16B and 16C advance into the position shown inFIG. 4B, the delivery end 20 of extrusion nozzle 14 becomes located tothe upstream end of the upstream mold block 16A (the first mold block).At this point of travel, mold block 16A trips a microswitch 17A and isopened to release the pipe which has just been molded within it. Thispipe 26 is still in softened condition and is supported in closed secondand third mold blocks 16B and 16C. Preferably a support mandrel 15 isprovided extending from the extrusion nozzle orifice 20 at least for ashort distance. Suitably, this short distance may be similar or nearlysimilar to the distance 26A, 26B and 26C backstepped by the mold blocks16A, 16B, 16C in a manner described below. Mold block 16A is disengagedfrom the forward carriage and backstepped into the position shown inFIG. 4C. This backstep movement takes place so that mold block 16A hasclosed again in the position shown in FIG. 4D before the softened pipein the unsupported section indicated as 26A has had time to sag or loseits molded shape. Closing may be initiated by microswitch 19A. The sizeof the backstep 26A has been shown in a series of FIG. 4 drawings asbeing appreciable. The sizes of the backstep shown in the drawings isfor ease of illustration. In fact, it is grossly exaggerated so that thesteady forward progression may be indicated clearly. The distance bywhich each of the mold blocks is backstepped may be small in comparisonwith the width of the corrugation or rib on pipe being molded. Thus avery small portion of pipe is unsupported or only supported by mandrelsupport 15 at any one time.

When the mold block 16A has reached the position shown in FIG. 4C andhas then been closed into the position shown in FIG. 4D, the next moldblock 16B is backstepped by a similar distance to support tube issuingfrom mold block 16A. Movement of mold blocks 16B and 16C is initiated bymicroswitches 17B, 17C and 19B, 19C in a similar manner to thatdescribed for mold block 16A. Thus as mold block 16A closes into theposition shown in FIG. 4D, mold block 16B opens and is disengaged fromthe forward carriage and is backstepped rapidly into the position shownin FIG. 4E.

It is sufficient for the purpose of the invention that only mold blocks16A and 16B be used so that the extrudate is supported at two points atall times. Thus, if only mold blocks 16A and 16B Are used the extrudatewill be supported by the extrusion nozzle and by mold block 16B whenmold block 16A is open and backstepping.

Although only two mold blocks may be used, it is probable that threemold blocks will be desirable for considerations of both support andminimum number of moving parts. Therefore, FIGS. 4F and 4G illustratethe further movement of mold block 16C in a similar manner to thatdescribed for mold block 16B, (or indeed for mold block 16A). It will beappreciated that more than three mold blocks may be used if desired.

Consideration as to the distance of backspace 26A is not only importantbecause tube must not sag or deform during this movement, but respectivemold blocks must re-engage molded tube so that their mold surfacesconform with already formed ribs or corrugations in the tube. Thus whenmold block 16B backsteps, it will close over tube which has already beenmolded. If it is not to deform such molding it must close on the pipewith considerable precision so that it grooves and crests mate exactlywith the corresponding crests and grooves of the pipe. Indeed, thisconsideration is probably of prime importance since mold block 16B mayre-mold any defects in the pipe which may have occurred due to undulysoft extrudate. Similar comments may be made in respect to mold block16C when it backsteps over section 26B of pipe. However, section 26Bwill have had the opportunity to cool and harden better than section26A.

Although the mold blocks 16A, 16B and 16C are shown in FIGS. 4A-4G asbeing of similar length, in practice it is likely that first mold block16A will be appreciably shorter than second mold block 16B. FIG. 5 showsa rough sketch of slightly more realistic comparative dimensions of thearrangement. Second mold block 16B usefully may carry services such ascooling fluid e.g. water or possibly air.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:
 1. Apparatus for formingthermoplastic pipe having a profiled surface, said apparatus comprisinga nozzle having an extrusion orifice feeding extrudate in a downstreamdirection to a mold path having a longitudinal axis extending from saidnozzle, first and second mold blocks on said mold path which areclosable to form the pipe and which are openable to release the pipe,the first mold block being in an upstream position relative to thesecond mold block, the mold blocks being moved together with one anotherand with the pipe in the downstream direction when the mold blocks areclosed, and the mold blocks being opened and sequentially moved in anupstream direction along an axis contiguous with the axis of the moldpath while the extrudate continues to move in the downstream directionwith the first mold block being moved in the upstream direction beforethe second mold block such that the mold blocks never pass one anotheron the mold path and the mold blocks then being reclosed around theextrudate at a position to maintain consistency of the profile of thepipe.
 2. Apparatus as claimed in claim 1, wherein said extrusion orificeis located within said first mold block during both the downstream andthe upstream movement of said first mold block.
 3. Apparatus as claimedin claim 1, wherein said first mold block is short relative to saidsecond mold block.
 4. Apparatus as claimed in claim 1, including a thirdmold block on said mold path downstream of said second mold block, saidfirst and second mold blocks always remaining upstream of said thirdmold block.
 5. Apparatus as claimed in claim 1, including a supportmandrel along the mold path onto which the extrudate is fed from saidnozzle.
 6. Apparatus as claimed in claim 5, wherein said support mandrelhas a length at least as long as the distance that said first mold blockis moved in the upstream direction.
 7. Apparatus as claimed in claim 1,including first and second mold block carriers which move in theupstream and the downstream direction and on which said first and secondmold blocks are respectively mounted, each mold block comprising a pairof diametric mold block halves which open and close relative to oneanother by sliding on their respective mold block carrier in a directiontransverse to the upstream and downstream movement of the mold blockcarrier.
 8. A method of forming thermoplastic pipe having a profiledsurface, said method comprising feeding an extrudate in a downstreamdirection from an extruder nozzle to a mold path having a longitudinalaxis extending from the nozzle with first and second mold blocks beinglocated on the mold path, the mold blocks being closable around theextrudate to form the pipe and being openable to release the pipe, thefirst mold block being in an upstream position relative to the secondmold block, moving the mold blocks together with one another and withthe extrudate in the downstream direction when the mold blocks areclosed, opening and sequentially moving the mold blocks in an upstreamdirection along an axis contiguous with the axis of the mold path whilethe extrudate continues to move in the downstream direction, the firstmold block being moved in the upstream direction before the second moldblock such that the mold blocks never pass one another on the mold path,and then reclosing the mold blocks around the extrudate at a position tomaintain consistency of the profiled surface of the pipe.